Microfiuidic systems for biochemical analysis are generally composed broadly of the following components: fluid channels to guide fluid flow, microvalves to regulate fluid flow, a pressure source to cause fluidic displacement, and biosensors to detect relevant parameter. Biosensors are biological sensors which, depending on their application, measure a particular parameter in a bodily fluid such as blood.
An analyte frequently in use for various diagnoses, blood, is a non-Newtonian fluid transporting oxygen, carbon dioxide, nutrients, salts, hormones, metabolites and various other components. The most important components of blood, from a rheological point of view, are plasma and RBC's (red blood cells). RBC's take up about half the volume of whole blood and significantly impact the flow characteristics of blood. Blood plasma thus accounts for about half the volume of whole blood. The plasma itself is about 95% (w/w) water. The rest is proteins, minerals, vitamins, glucose etc.
Quite often, biosensors need to deal with irrelevant or unwanted components in the sensed solution. For example, in a microfluidic device to measure blood glucose level, the sensor where the electrochemical reaction takes place frequently experiences interference from microparticles like red blood cells, thereby reducing its efficiency and possibly increasing reaction time. In other cases, the microparticles might cause interference during detection by optical means due to obstruction of projected light. The separation of these microparticles from the remainder of the suspension solution is thus often desirable.
The most common methods in use to achieve separation of bioparticles, in lab-on-a-chip or μTAS systems, have been physical filtration and less commonly, centrifugation. U.S. patents USRE31688, U.S. Pat. No. 5,914,042, U.S. Pat. No. 5,906,570, and U.S. Pat. No. 4,619,639, incorporated in their entirety by reference herein, describe some of the different types of membrane filtration methods. The underlying principle in this technique is to cause separation of blood into its constituents by making it flow through a membrane filter with a number of pores having micrometer-range dimensions. According to the purpose of the device, the pore size of the membranes is varied, ranging from approximately, 0.5 micron diameter to 5 micron diameter. In the case of a very small pore size, some positive pressure might be required to make the solution cross the membrane. There are a number of membrane materials and compositions in use. Patent applications WO9839379A1, WO9720207A1, and WO7901121A1 describe some more membrane methods.
The other major technique for separation is by using centrifugal force applied to the suspension solution causing separation depending on the specific mass and gravities of the particles which compose the solution. The solution is contained in a chamber and rapidly rotated at high angular speed, which causes application of the centrifugal force. U.S. Pat. No. 6,315,707 and European patent application EP0520185A1 describe centrifugation methods. Another method, Field Flow Fractionation is mentioned in U.S. patent application UA20040011651A1. Here, the solution containing the microparticles is made to flow through a separation channel or chamber where an applied electric or thermal field causes fractionation or separation of the particles due to the field gradient across the width of the channel. Depending on their charge or temperature response, particles are drawn to the top or bottom of the channel and the remaining solution can be suitably extracted. U.S. patent application US20040018611A1 deals with yet another method for particle separation utilizing the effects of a High Gradient Magnetic Separation (HGMS). Particles are either tagged with magnetic particles, or simply subjected to a magnetic field and thus separated based on their magnetic response. Another technique to efficiently separate blood cells from plasma is to cause aggregation of blood cells by exposure of whole blood to an ultrasonic standing wave as described in Cousins et al, “Clarification of plasma from whole human blood using ultrasound,” Ultrasonics, Vol. 38, 2000. When this is done, the cells concentrate into clumps at radial separations of half wavelengths. The clumps grow in size and sediment under gravity and a distinct plasma/cell interface forms as cells sediment.
However, centrifugation is too bulky for point-of-care handheld detection equipment. It is laboratory specific and time-consuming. Magnetic separation and Field flow fractionation require the use of specialized equipment as does Ultrasonic separation. These techniques are generally more expensive than the other methods. Membrane filtration is suitable for disposable, point-of-care applications but the inherent problems have been aggregation and fragility of materials. The fabrication of the membranes and the subsequent integration of membranes into existing devices can be a complicated process. It is also relatively more time-consuming. Furthermore, membrane filtration techniques are also more expensive and require high pressures and/or specialized fluid driving.
No known technique has been able to achieve the characteristics of bioparticle separation with an easy and low-cost method.